1. Field of the Invention
The present invention relates to a servo field on a disk of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
In general there are two types of recording heads used in commercial disk drive, horizontal recording heads and perpendicular recording heads. Perpendicular recording heads magnetize the disk in a direction perpendicular to the outer surface of the disk.
Data is stored on tracks located on the surfaces of the disks. Each track typically contains a plurality of sectors. A sector may have a servo field that is used to center the head on the track. The servo field contains a number of spaced apart servo bursts. By way of example, the servo field may include A and B servo burst. The amplitude of the servo bursts will vary depending on the position of the head relative to each burst. A position error signal is generated from the servo bursts. The position error signal is used to center the head on the track.
The servo burst are typically written with a servo writer during the manufacturing process of the disk drive. The writing of servo bursts is a relatively inaccurate process. The servo burst may overlap to create inaccurate servo data. For example, an A servo burst may extend into an area normally associated with the B servo burst. This may result in an inaccurate servo burst profile.
FIG. 1 shows a servo field with an A servo burst 1 and a B servo burst 2. The A servo burst 1 has erase bands 3. The erase bands 3 insure that the magnetized portion of the A burst 1 is not associated with the B servo burst 2.
As shown by the graphs in FIGS. 2 and 3 the width of the erase bands varies between the inner diameter and outer diameter of the disk for a disk drive that utilizes a perpendicular recording head. The problem is particularly acute for bi-directional servo writing. The width of the erase band can also vary as a function of servo frequency (spindle motor speed) and write current. These variations can create a burst profile 5 that has a flat portion 6 as shown in FIG. 1. In general, it is undesirable to have burst profiles with flat portions. It would be desirable to provide a scheme that compensates for the variations in the width of the erase bands to minimize or eliminate any flat portions of the burst profiles.